In reactive current compensators it is known to control compensating current magnitude by either variation of compensating reactance or by direct control of current magnitude in fixed reactors. It is also known to provide fixed compensating reactance of one characteristic, as capacitive, and counteracting compensating reactance of opposite characteristic, as inductive, with means for controlling current magnitude in the counteracting reactance, thereby to control the net amount of reactive compensating current. A preferred reactive current compensating apparatus utilizing fixed compensating capacitors and fixed counteracting inductors with conduction angle control of inductive compensating current magnitude is described and claimed in U.S. Pat. No. 3,936,727-Lezan and Kelley. When such apparatus is used with a primarily inductive load the variable current inductive compensating circuit normally requires a maximum current capability only equal to or less than that of the capacitive compensating circuit.
In operation of certain erratic loads however, as three phase electric arc furnaces, certain random conditions of load circuit unbalance may require momentarily a net inductive compensating current in a single power circuit phase only. This requirement may vary in random manner from phase to phase but ordinarily exists in only one phase at any instant. When controlled compensating inductors are used to variably counteract fixed compensating capacitors as described above it is of course possible to provide for a net inductive compensating current in any phase by designing the inductors in all phases with a maximum current capability greater than that of the corresponding compensating capacitors. However, where maximum inductive compensating current is required in only one phase at any one time such a design is unduly expensive. It is desirable therefore that means be provided for increasing the maximum reactive current capability of any one selected phase in a variable multiphase reactive current compensator without increasing the current carrying capability of all phases of the compensator.